Sr doped lanthanum manganites La_1-xSr_xMnO_3
show colossal magnetoresistance effects that strongly depend
on alloy composition. Many recent studies show the existence
of charge, orbital, and spin ordering, and a number of
anomalies have been observed in transport or magnetic
properties. These properties have been generally been
explained assuming that the sample composition is
homogeneous through the entire sample. However, there is
always the possibility for the non-random distribution of La
and Sr atoms among the allowed sites with possible
tendencies towards ordering or phase separation of the La
and Sr atoms. Here we report XAFS studies of Sr K-edge and
La K-edge to determine the local coordination of Sr around
Sr and La atoms as a function of alloy composition. Our
results do indeed indicate non-random site occupation for
the Sr and La ions, and also show interesting anomalies in
local lattice distortion as a function of alloy composition.
[A24.002] The metal-insulator transition in CMR manganites: a strange kind of percolation
Simon J. L. Billinge (Michigan State University)
The nature of the metal-insulator (MI) transition in
colossal magnetoresistant manganites is still the subject of
lively debate. Of note recently is the observation that both
the ferromagnetic metallic and the paramagnetic insulating
states below and above the MI transition, resepectively, are
quite inhomogeneous. In this picture, the sample
microscopically phase separates into metallic and insulating
domains. The metal-insulator transition can then be
understood in the framework of a percolation transition.
Because of the strong coupling of the electronic system to
the lattice through the Jahn-Teller distortion, this
metal-insulator transition can be quantitatively studied
using local structural probes such as the atomic pair
distribution function (PDF) method. We discuss the nature of
the MI transistor transition and show that it is not a
conventional percolation transition and important physics
beyond this simple percolation picture are necessary to
explain the observations.
[A24.003] Recent Developments in the Phase Separation Theory of Manganites
Elbio Dagotto, Adriana Moreo, Matthias Mayr, Jan Burgy, Adrian Feiguin (National High Magnetic Field Lab and Department of Physics, Florida State University, Tallahassee, FL 32306, USA)
The present status of the mixed-phase scenario for manganites will be discussed(E. Dagotto, T. Hotta, and A. Moreo, to appear in Physics Reports.). Results include (a) the low-temperature coexistence of large equal-density ferromagnetic and antiferromagnetic clusters, and concomitant percolative process, when disorder influences on first-order transitions of the non-disordered limit(A. Moreo et al.), Phys. Rev. Lett. 84, 5568 (2000). , (b) calculations of resistivity vs temperature using a random-resistor network(M. Mayr et al.), Phys. Rev. Lett. 85, issue 26, Dec.2000. in agreement with experiments, (c) indications of a novel quantum critical behavior in the percolative regime (J. Burgy et al.), in preparation., and (d) the possibility of a new temperature scale T where clusters start forming above the Curie temperature.
[A24.004] Charge Ordering and Its Fluctuation in Manganites
S-W. Cheong (Department of Physics amp; Astronomy, Rutgers University and Bell Laboratories, Lucent Technologies), K. H. Kim, M. Uehara, B. G. Kim, V. Podzorov, M. Gershenson (Department of Physics amp; Astronomy, Rutgers University), C. H. Chen (Bell Laboratories, Lucent Technologies)
In doped Mott insulators, charge carriers tend to
self-organize in such a way as to form stripe patterns at
low temperatures. In the case of doped manganites, orbital
degrees of freedom associated with Mn^3+ can be involved
in the striped ordering. This striped charge/orbital
ordering at low temperatures in manganites is associated
with various length scales. Particularly, recent results
indicate the presence of very-short-range or dynamic
correlation of charge/orbital ordering at high temperatures,
far above any phase transition temperatures. This
significant fluctuation is directly associated with the
origin of the colossal magnetoresistance effect.
[A24.005] Towards a test of the "bistripe" and Wigner Crystal models of charge ordering: approaches to crystal growth of LCMO (x=2/3)
John Mitchell (Materials Science Divsion, Argonne National Laboratory), Hong Zheng (Materials Science Divsions, Argonne National Laboratory), Dean Miller, Dimitri Argyriou (Materials Science Division, Argonne National Laboratory)
The mode of charge ordering of Mn(III) and Mn(IV) ions in
the heavily doped (x>0.5) LCMO system remains in
controversy. Lattice images of samples from this region of
the phase diagram indicate a fascinating "bistripe"
arrangement in which Mn(III) pairs interleave with Mn(IV)
regions. The spacing of these bistripes is determined by the
Ca concentration. In contrast, neutron diffraction on bulk
samples indicates a more homogeneous distribution of the
dilute Mn(III) ions, whose arrangement into a Wigner lattice
minimizes the Coulomb energy of the ensemble. Refinement of
the powder neutron data relied on comparing intensity of
weak superlattice reflections to model predictions. To
reconcile these disparate models, we have undertaken growth
of single crystals of the x=2/3 composition in the solid
solution. In this talk we discuss the issues facing the
successful crystal growth of this system including process
parameters that impact the crystal growth process.
[A24.006] Raman Scattering and Ellipsometric Studies of the Charge-Ordered Phase of (Bi,Ca)MnO3*
S. L. Cooper (Department of Physics and Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana-Champaign)
We have studied the dramatic effects of charge ordering on the charge-, spin-, and lattice-dynamics of the (Bi,Ca)MnO3 system using spectroscopic ellipsometry and Raman scattering. By probing the dielectric anisotropy at different temperatures and energies, spectroscopic ellipsometry has allowed us to study how charge-ordering influences the charge- and orbital degrees of freedom on various length scales. Among other effects, we observe evidence for significant charge-stripe fluctuations above the charge-ordering transition, and for different temperature dependencies associated with charge- and orbital-ordering. The temperature evolution of the dielectric anisotropy also exhibits interesting similarities with the development of the phonon spectrum probed by Raman scattering, which reveals evidence for anisotropic lattice dynamics and the activation of forbidden phonon modes below the charge-ordering transition. Additionally, we observe in the antiferromagnetic charge-ordered phase the development of a quasielastic light scattering response having a distinctive T1g symmetry. This unusual scattering symmetry transforms like the spin-chirality operator (S1*S2xS3), and thus this quasielastic response betrays the presence of strong magnetic or chiral fluctuations at finite temperatures in the antiferromagnetic charge-ordered phase. As will be discussed, among the possible sources of this anomalous response are (i) fluctuations of the core spins associated with a canted antiferromagnetic or spin-chiral phase, and (ii) fluctuations associated with closed-loop charge currents, arising from the strong constraints placed on conduction by the complex spin-texture of the Neel state and the double-exchange hopping mechanism.
*This work was supported in part by the Department of Energy
under DEFG02-96ER4539, and was performed in conjunction with
S. Yoon, M. Ruebhausen, K. H. Kim, and S-W. Cheong.
[A24.007] Phase segregation in manganite perovskites: FM clusters at the crossover between two AFM states
Chris Ling (Argonne National Laboratory), John Neumeier (Florida Atlantic University), Dimitri Argyriou (Argonne National Laboratory)
The CMR perovskite La_1-xCa_xMnO_3 has been relatively neglected in high-x region. Magnetic susceptibility measurements suggest the presence of small FM clusters in the crossover region between the Type-C and Type-G AFM states. A phase transition intimately associated with the C-AFM (but not G-AFM) state has allowed us to determine with which AFM phase the FM clusters are associated, via a combination of synchrotron XRD and neutron powder diffraction. The high-symmetry phase was found to contain a FM component in addition to G-AFM, but the low-symmetry phase was found to contain no FM component in addition to C-AFM. Furthermore, there is evidence that below T_N(G), the low-symmetry phase associated with C-AFM is slowly colonized by G-AFM, similar to our recent observations of the analogous pseudo-2D system La_2-2xSr_1+2xMn_2O_7.
This work was supported by the Office of Science, US DOE,
under contract W-31-109-ENG-38 (CL, DA) and by the NSF under
contract DMR 9982834 (JN).
[A24.008] Mesoscopic, Non-equilibrium Fluctuations of Inhomogeneous Electronic States in Manganites
V. Podzorov (Serin Physics Laboratory, Rutgers University, Piscataway, NJ 08854), C. H. Chen (Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974), M. E. Gershenson, S-W. Cheong (Serin Physics Laboratory, Rutgers University, Piscataway, NJ 08854)
By using the dark-field real-space imaging technique of
transmission electron microscopy (TEM), we have observed
slow 200 Å-scale fluctuations of charge-ordered (CO) phase
in mixed-valent manganites under a strong electron beam
irradiation. These unusual fluctuations of the CO phase are
related to the switching-type fluctuations of electrical
esistivity, which were found to be as large as several
percents. Systematic analysis indicates that this slow
dynamics with a time scale of the order of seconds
originates from non-equilibrium processes, and a
meta-stable, charge-"disordered" insulating phase, plays an
essential role in the observed fluctuations.
[A24.009] Thermodynamics and Kinetics of Mesoscopic Phase Fluctuations in a Bulk CMR Crystal
Frank Hess, Robert Merithew, Mike Weissman (University of Illinois at Urbana-Champaign), Y. Tokura, Y. Tomioka (Joint Research Center for Atom Technology)
We observe discrete two-state conductance fluctuations at
the CMR transition in a bulk La_0.66Ca_0.33MnO_3
single crystal (R.D Merithew et. al. PRL \textbf84, 3442
(2000)). The magnetic moment and entropy differences between
the two resistance states are determined from the
temperature and field dependences of the switchers' duty
cycles. The resulting entropy to magnetic moment ratios are
similar to results obtained from conventional heat capacity
and magnetization measurements. Analysis of the switchers'
transition rates show the barrier state has moment and
entropy intermediate between the two stable end states, as
would be natural for a domain switching between a
ferromagnetic, low entropy phase and a paramagnetic, high
entropy phase. Switchers which persist after the sample has
undergone temperature cycles into the insulating phase and
back indicate the detailed structure of the mixed phase is
partially determined by quenched disorder. This work was
supported by NSF DMR 99-81869.
[A24.010] Optical investigation of La_1-xCa_xMnO_3 (x=0.48, 0.5, and 0.52)
K. H. Kim, T.W. Noh (School of Physics, Seoul National University, Seoul 151-742, Korea), S.-W. Cheong (Dept. of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854 and Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974)
We investigated temperature (T)-dependent optical conductivity spectra \sigma (ømega ) of La_1-xCa_xMnO_3 (% x=0.48, 0.5, and 0.52). \sigma (ømega ) of x=0.48 and 0.50 samples exhibit two absorption bands in a mid-infrared region at 10 K, showing clearly a coexistence of both ferromagnetic metallic and charge orderded (CO) insulating domains in these compounds. Doping dependence of \sigma (ømega ) at 10 K reveals that charge gap due to the CE type CO, stabilized in La_1-xCa_xMnO_3 near x=0.5, is \sim 0.5 eV, which is quite higher than a previously reported value of \sim 0.1 eV. On the other hand, above T_CO, all the compounds have a broad mid-infrared absorption band centered around 1.0 eV, accompanied by a pseudogap feature at a lower frequency region. The pseudogap behavior of x=0.5 becomes more evident than those of x=0.48 and 0.52, suggesting that a short-range CO correlation persisting at a high T region can be closely related to the observed pseudogap behavior.